The method of this invention pertains to the recovery of soda ash, as sodium carbonate monohydrate, from aqueous sodium carbonate solutions also containing sodium sulfate and sodium chloride.
The recovery of soda ash from aqueous sodium carbonate solutions that also contain sodium sulfate, sodium chloride and other salts has been described in connection with treatment procedures applicable to complex natural brines.
U.S. Pat. No. 1,853,275 issued to Houghton et al. describes the selective recovery of sodium carbonate decahydrate from Owens Lake brines containing carbonates, sulfates, chlorides and borates of potassium and sodium. Such brines are first pretreated by adjusting their temperature to about 35.degree. C., in the presence of excess sodium chloride and sodium carbonate, to crystallize burkeite (Na.sub.2 CO.sub.3.2Na.sub.2 SO.sub.4) and thereby reduce the sulfate concentration in the brine. The low sulfate brine is then chilled to crystallize sodium carbonate decahydrate, which is recovered. To remove all traces of sulfate, the recovered decahydrate is recrystallized as sodium carbonate monohydrate.
U.S. Pat. No. 2,193,817, issued to Houghton, discloses an improvement in the Houghton et al. '275 decahydrate process in which sodium carbonate heptahydrate is crystallized at low temperature from NaCl-saturated brines to minimize co-crystallization of sulfate with the hydrated carbonate product.
Several drawbacks are associated with the low temperature cooling crystallizations utilized by these prior art processes. Refrigeration requirements may be costly, and particularly for NaCl- and Na.sub.2 SO.sub.4 -containing carbonate solutions that are hot, cooling such solutions is economically inappropriate. Recovery of a dense soda ash requires that the recovered decahydrate or heptahydrate solids be recrystallized, at elevated temperature, to form sodium carbonate monohydrate or anhydrous sodium carbonate.
Recovery efficiencies of sodium carbonate by these procedures are not high: the Houghton et al. '275 method fails to recover any sodium carbonate values contained in the burkeite (Na.sub.2 CO.sub.3.2Na.sub.2 SO.sub.4) that is initially crystallized, and sodium carbonate yields in the Houghton '817 method are limited by the high NaCl content required for the crystallization solution.
The method of the present invention provides for the efficient recovery of a dense soda ash, as relatively pure sodium carbonate monohydrate, from Na.sub.2 SO.sub.4 - and NaCl-containing sodium carbonate solutions.